A number of different products include audio circuitry, such as an audio amplifier, together with one or more loudspeakers and/or connections for driving one or more loudspeakers of a peripheral apparatus such as a headset. In some instances the loudspeaker(s) chosen will be robust enough and large enough to handle the maximum power level at which the amplifier could drive signals continuously into it, even under the worst case environmental conditions, for instance maximum supply voltage, maximum ambient temperature etc. However having robust enough loudspeakers is not always economical, and for portable devices in particular the desire is typically to make the speaker as small and light as possible. This can potentially lead to the audio drive circuitry overloading the loudspeaker. One particular problem is thermal overload of the loudspeaker.
A typical loudspeaker comprises a diaphragm which is driven by a voice coil supported relative to a magnet. In use, typically, an analogue audio drive signal is applied to the voice coil to drive the loudspeaker. High levels of audio signal may give rise to high current levels in the voice coil, which may cause it to heat up due to ohmic losses associated with the electrical resistance of the coil. Excessive voice coil temperature may give rise to reliability problems, for example the glue attaching component parts of the mechanism may soften or melt, or the permanent magnet may be demagnetized or the ferro-magnetic properties of the core may degrade.
The audio amplifier circuitry may therefore be controlled by thermal protection control circuitry to limit the output power it can deliver to the loudspeaker so that the maximum power dissipated in the voice coil does not cause the voice coil temperature to exceed some specified safe limit.
However the maximum power that may be allowed to be dissipated in the voice coil will be dependent on the ambient temperature. It will be appreciated that heat generated in the voice coil will flow to the surrounding environment via various thermal paths (with associated thermal resistances) to the environment which will be at an ambient temperature. At a hot ambient temperature, less power can be dissipated before the voice coil temperature rises to a maximum limit compared to a cooler ambient. The output power limit may thus be set to a value appropriate for a maximum likely or specified temperature. However this does mean that the output power may be unnecessarily limited when operating with a more usual or even unusually low ambient temperature.
To mitigate this, the voice coil temperature may be sensed and the power applied adjusted to control the measured coil temperature. However the gain of the control loop is preferably designed to be low, to avoid sudden audible changes in power level. Thus the maximum power is still somewhat dependent on other influences such as ambient temperature variations, and may still be less than the maximum actually allowable.